The guys who run track days probably don't switch rotors every time because they'll burn through a set of pads in no time. However, for daily driving I always replace rotors and pads at the same time. Costs a bit, but the outcome is always no shakes, no squeals, no worries. The pads want to seat into a fresh machined surface. Can you get away with just a pad slap? Sure, but I'd rather do it all and be done.